Messages in this thread | | | Date | Wed, 7 Jan 2009 19:12:33 +0100 | From | Ingo Molnar <> | Subject | Re: [BUG] 2.6.28-git LOCKDEP: Possible recursive rq->lock |
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* Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> wrote:
> * Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> [2009-01-07 22:01:00]: > > > * Peter Zijlstra <a.p.zijlstra@chello.nl> [2009-01-07 15:28:57]: > > > > > On Wed, 2009-01-07 at 19:50 +0530, Vaidyanathan Srinivasan wrote: > > > > * Peter Zijlstra <a.p.zijlstra@chello.nl> [2009-01-07 14:12:43]: > > > > > > > > > On Wed, 2009-01-07 at 17:59 +0530, Vaidyanathan Srinivasan wrote: > > > > > > > > > > > ============================================= > > > > > > [ INFO: possible recursive locking detected ] > > > > > > 2.6.28-autotest-tip-sv #1 > > > > > > --------------------------------------------- > > > > > > klogd/5062 is trying to acquire lock: > > > > > > (&rq->lock){++..}, at: [<ffffffff8022aca2>] task_rq_lock+0x45/0x7e > > > > > > > > > > > > but task is already holding lock: > > > > > > (&rq->lock){++..}, at: [<ffffffff805f7354>] schedule+0x158/0xa31 > > > > > > > > > > > > other info that might help us debug this: > > > > > > 1 lock held by klogd/5062: > > > > > > #0: (&rq->lock){++..}, at: [<ffffffff805f7354>] schedule+0x158/0xa31 > > > > > > > > > > > > stack backtrace: > > > > > > Pid: 5062, comm: klogd Not tainted 2.6.28-autotest-tip-sv #1 > > > > > > Call Trace: > > > > > > [<ffffffff80259ef1>] __lock_acquire+0xeb9/0x16a4 > > > > > > [<ffffffff8025a6c0>] ? __lock_acquire+0x1688/0x16a4 > > > > > > [<ffffffff8025a761>] lock_acquire+0x85/0xa9 > > > > > > [<ffffffff8022aca2>] ? task_rq_lock+0x45/0x7e > > > > > > [<ffffffff805fa4d4>] _spin_lock+0x31/0x66 > > > > > > [<ffffffff8022aca2>] ? task_rq_lock+0x45/0x7e > > > > > > [<ffffffff8022aca2>] task_rq_lock+0x45/0x7e > > > > > > [<ffffffff80233363>] try_to_wake_up+0x88/0x27a > > > > > > [<ffffffff80233581>] wake_up_process+0x10/0x12 > > > > > > [<ffffffff805f775c>] schedule+0x560/0xa31 > > > > > > > > > > I'd be most curious to know where in schedule we are. > > > > > > > > ok, we are in sched.c:3777 > > > > > > > > double_unlock_balance(this_rq, busiest); > > > > if (active_balance) > > > > >>>>>>>>>>> wake_up_process(busiest->migration_thread); > > > > > > > > } else > > > > > > > > In active balance in newidle. This implies sched_mc was 2 at that time. > > > > let me trace this and debug further. > > > > > > How about something like this? Strictly speaking we'll not deadlock, > > > because ttwu will not be able to place the migration task on our rq, but > > > since the code can deal with both rqs getting unlocked, this seems the > > > easiest way out. > > > > Hi Peter, > > > > I agree. Unlocking this_rq is an easy way out. Thanks for the > > suggestion. I have moved the unlock and lock withing the if > > condition. > > > > --Vaidy > > > > sched: bug fix -- do not call ttwu while holding rq->lock > > > > When sched_mc=2 wake_up_process() is called on busiest_rq > > while holding this_rq lock in load_balance_newidle() > > Though this will not deadlock, this is a lockdep warning > > and the situation is easily solved by releasing the this_rq > > lock at this point in code > > > > Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> > > > > diff --git a/kernel/sched.c b/kernel/sched.c > > index 71a054f..703a669 100644 > > --- a/kernel/sched.c > > +++ b/kernel/sched.c > > @@ -3773,8 +3773,12 @@ redo: > > } > > > > double_unlock_balance(this_rq, busiest); > > - if (active_balance) > > + if (active_balance) { > > + /* Should not call ttwu while holding a rq->lock */ > > + spin_unlock(&this_rq->lock); > > wake_up_process(busiest->migration_thread); > > + spin_lock(&this_rq->lock); > > + } > > > > } else > > sd->nr_balance_failed = 0; > > > Hi Peter and Ingo, > > The above fix seem to have fixed the lockdep warning. Please include > in sched-tip for further testing and later push to mainline.
already in tip/sched/urgent, thanks guys!
Ingo
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